"Slushing" is just another term for updating the firmware. "Backslushing" means going back to a previous version of the firmware.
Is there a specific reason for asking?
The only reason for slushing the boards is if you have installed a version of the software that is incompatible with your hardware. Slushing up shouldn't be an issue (if you have compatible hardware!) but backslushing is generally not recommended.
Does that help?
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